Reduction of punch-through disturb during programming of a memory device

ABSTRACT

In one or more of the disclosed embodiments, a punch-through disturb effect in a memory device can be reduced by biasing a selected word line at a program voltage to program a selected memory cell, biasing word lines on the drain side of the series string with a V pass  voltage, turning off an adjacent memory cell to the selected memory cell, and biasing remaining word lines on the source side of the turned-off memory cell with a V low  voltage that is less than V pass .

TECHNICAL FIELD

The present disclosure relates generally to semiconductor memory andmore particularly to non-volatile memory devices.

BACKGROUND

Flash memory devices have developed into a popular source ofnon-volatile memory for a wide range of electronic applications. Flashmemory devices typically use a one-transistor memory cell that allowsfor high memory densities, high reliability, and low power consumption.Changes in threshold voltage of the cells, through programming of chargestorage nodes such as floating gates or trapping layers or otherphysical phenomena, determine the data value of each cell. Common usesfor flash memory and other non-volatile memory include personalcomputers, personal digital assistants (PDAs), digital cameras, digitalmedia players, digital recorders, games, appliances, vehicles, wirelessdevices, mobile telephones, and removable memory modules, and the usesfor non-volatile memory continue to expand.

Two common types of flash memory array architectures are the “NOR” and“NAND” architectures. These architectures are named for the resemblancethat the basic memory cell configuration of each architecture has to abasic NOR or NAND gate circuit, respectively.

In the conventional NOR array architecture, floating gate memory cellsof the memory array are arranged in a matrix. The gates of each floatinggate memory cell of the array matrix are connected by select lines,conventionally referred to as “word lines”, and their drains areconnected to transfer lines that are conventionally referred to as bitor digit lines. Memory cells having their control gates connected to acommon select line are considered to be a “row” of memory cells whilememory cells having their drains connected to a common transfer line areconsidered to be a “column” of memory cells. The source of each floatinggate memory cell is typically connected to a common source line. The NORarchitecture floating gate memory array is accessed by a row decoderactivating a row of floating gate memory cells by selecting the wordline connected to their control gates. The row of selected memory cellsthen place their stored data values on the bit lines by flowing adiffering current if in a programmed state or not programmed state fromthe connected source line to the connected bit line.

A NAND architecture arranges its array of non-volatile memory cells in amatrix of rows and columns so that the gates of each non-volatile memorycell of the array are coupled by rows to word lines. However, unlikeNOR, each memory cell is not directly coupled to a source line and a bitline. Instead, the memory cells of the array are arranged together instrings, typically of 8, 16, 32, or more each, where the memory cells inthe string are coupled together in series, source to drain, between acommon source line and a column bit line. It is noted that othernon-volatile memory array architectures exist, including, but notlimited to AND arrays, OR arrays, and virtual ground arrays.

One problem with programming in a NAND memory array is program disturb.Program disturb refers to the increase of the threshold voltages ofmemory cells in a bit line and/or a word line containing a memory cellbeing programmed. The threshold voltage increase is a result of theprogramming voltage that is applied to the cell being programmedaffecting other cells coupled to the bit line/word line as well. Programdisturb can result in an unprogrammed cell being programmed or aprogrammed cell changing states.

One method used to reduce program disturb is a boosting scheme thatbiases unselected word lines with a program inhibit voltage. Forexample, the unselected word lines in a NAND series string of memorycells can be biased with 10V. The unselected word lines couple tounselected bit lines causing a voltage to exist in the channel of theunselected bit lines. This tends to reduce the disturb condition.

A problem with this scheme is that punch-through can occur on nearbyword lines in the same series string if these cells are turned off. Theeffect of punch-through is to increase the threshold voltage for thecells coupled to the affected word lines. Typically punch-through hasthe greatest affect on the source-side word lines of the selected wordline.

For the reasons stated above, and for other reasons which will becomeapparent to those skilled in the art upon reading and understanding thepresent specification, there is a need in the art for reducing apunch-through disturb effect in a memory device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic diagram of one embodiment of a portion of amemory array.

FIG. 2 shows a flowchart of one embodiment of a method for programmingthat incorporates a self-boosting method in a memory device.

FIG. 3 shows one embodiment of a cross-sectional view of a series stringof memory cells in accordance with the method for programming of FIG. 2.

FIG. 4 shows a block diagram of one embodiment of a memory system thatincorporates the back pattern compensation of the present disclosure.

DETAILED DESCRIPTION

In the following detailed description of the present embodiments,reference is made to the accompanying drawings that form a part hereof,and in which is shown by way of illustration specific embodiments inwhich the embodiments may be practiced. These embodiments are describedin sufficient detail to enable those skilled in the art to practice theinvention, and it is to be understood that other embodiments may beutilized and that process, electrical or mechanical changes may be madewithout departing from the scope of the present disclosure. Thefollowing detailed description is, therefore, not to be taken in alimiting sense.

FIG. 1 illustrates a schematic diagram of a portion of a NANDarchitecture memory array comprising series strings of non-volatilememory cells. The array is comprised of an array of non-volatile memorycells 101 (e.g., floating gate) arranged in series strings 104, 105 thatcan also be referred to as columns. Each of the cells 101 are connecteddrain to source in each series string 104, 105. A word line WL0-WL31that spans across multiple series strings 104, 105 is connected to thecontrol gates of each memory cell in a row in order to provide differentbiasing levels to each memory cell to enable different operations (i.e.,program, read, erase) in response to a connected bit line. The bit linesBL1, BL2 are connected to sense amplifiers (not shown) that detect thestate of each cell by sensing current on a particular bit line.

Select lines, such as word lines WL0-WL31, select the individual memorycells in the series strings 104, 105 to be written to (i.e., programmed)or read from and operate the remaining memory cells in each seriesstring 104, 105 in a pass through mode. Each series string 104, 105 ofmemory cells is coupled to a source line 106 by a source select gate116, 117 and to an individual bit line BL1, BL2 by a drain select gate112, 113. The source select gates 116, 117 are controlled by a sourceselect gate control line SG(S) 118 connected to their control gates. Thedrain select gates 112, 113 are controlled by a drain select gatecontrol line SG(D) 114.

Each memory cell can be programmed as a single level cell (SLC) ormultilevel cell (MLC). Each cell's threshold voltage (V_(t)) isindicative of the data that is stored in the cell. For example, in anSLC, a V_(t) of 0.5V might indicate a programmed cell while a V_(t) of−0.5V might indicate an erased cell. The MLC may have multiple V_(t)windows (i.e., range of V_(t) voltages) that each indicate a differentstate. Multilevel cells take advantage of the analog nature of atraditional flash cell by assigning, for example, a bit pattern to aspecific voltage range stored on the cell. This technology permits thestorage of two or more bits per cell, depending on the quantity ofvoltage ranges assigned to the cell.

FIG. 2 illustrates a flowchart of one embodiment of a method forprogramming a memory device that incorporates a self-boosting method.The programming is typically performed on a per memory block (i.e., agroup of a predetermined quantity of memory cells) basis but the presentembodiments are not limited to any one programming scheme. The biasingsteps are not required to be performed in any particular order forproper operation of the disclose method.

The source-side unselected word lines are biased 201 according to one ofthe four embodiments shown in the following table. Similarly, thedrain-side unselected word lines are also biased 203 according to thefollowing table. The selected word line comprising the memory cells tobe programmed is biased at V_(pgm) 205.

WL0 . . . WL(N − 4) WL(N − 3) WL(N − 2) WL(N − 1) WLN WL(N + 1) . . .WL31 1 V_(low) V_(low) V_(low) V_(low) GND V_(pgm) V_(pass) V_(pass) 2V_(low) V_(low) V_(low) GND biasL V_(pgm) V_(pass) V_(pass) 3 V_(low)V_(low) GND biasL V_(pass) V_(pgm) V_(pass) V_(pass) 4 V_(low) V_(low)V_(low) GND biasL V_(pgm) biasH V_(pass)

Approximate voltage ranges for the biasing of the self-boosting schemesillustrated in the table include V_(low) having a range of 0V to 6V,V_(pass) having a range of 9V to 10V, V_(pgm) having a range of 15V to23V, both “biasL” and “biasH” are typically about equal and rangebetween V_(low) and V_(pass) (i.e., 6V to 10V). Both “biasL” and “biasH”are voltages that improve self-boost where “biasH” is used on thedrain-side of the series string and “biasL” is used on the source-sideof the series string. These voltage ranges are for purposes ofillustration only as the present embodiments are not limited to anyparticular voltage ranges.

The table shows that at least one word line on the source-side of theselected word line (i.e., WLN) is biased at ground potential. The groundpotential turns off the memory cells coupled to that word line.Additionally, a memory cell is off whenever the V_(t) of the cell isgreater than 0. If the V_(t) of the cell is negative, the cell is notoff and punch-through is not a problem. The word lines on thesource-side of the “turned-off” word line are biased at V_(low). TheV_(low) bias reduces the potential difference across the turned off wordline, thus reducing the punch-through disturb effect.

To improve the self-boost to the selected cell on the selected word line(WLN) and reduce the punch-through effect further, the memory cell onthe adjacent series string, that is adjacent to the “turned-off” cell onthe selected series string, should also be turned-off. As an example ofoperation of embodiment 2 of the table, referring again to FIG. 1, WL30is assumed to be equivalent to WLN. The memory cell 100 coupled to BL1is the selected cell 100. Thus, according to embodiment 2 of the table,WL30 would be biased at Vpgm, WL29 (i.e., WL(N−1)) would be biased atbiasL, WL28 (i.e., WL(N−2)) would be biased at GND, and the remainingword lines of BL1 would be biased at V_(low). BL1 would be biased at 0Vto enable the bit line for programming and BL2 would be biased at V_(CC)in order to inhibit the adjacent bit line and to turn off the memorycell 130 adjacent to the turned-off cell 131 of BL1. WL31 would bebiased at V_(pass).

In one embodiment, the programming voltage, V_(pgm), is a series ofincrementally increasing programming pulses. For example, if the firstprogramming pulse does not program the memory cell or cells to thetarget threshold voltage, V_(pgm) is increased by a step voltage (e.g.,1V) and the new programming pulse is applied to the selected word lineagain. The incrementing is repeated until the memory cell is programmedto the target threshold voltage.

The bit lines of the memory block being programmed are also biased in anenable/inhibit manner. The bit lines coupled to the cells to beprogrammed are typically biased at 0V while the inhibited bit lines aretypically biased at V_(CC).

Referring again to FIG. 2, a program verify operation is then performed207 to determine if the cell or cells to be programmed have reached thetarget threshold voltage. The program verify is comprised of biasing theselected word line containing the cell to be verified with a verifyvoltage. A typical verify voltage can be 0.5V. The remaining unselectedword lines can be biased with a pass voltage of approximately 5.8Vduring the verify operation. Additionally, the bit line is also biasedso that it is enabled for verification. The select gate source andselect gate drain transistors are biased so that they are turned on toallow the selected series string to conduct to the bit line.

If the verification operation determines that the memory cell is notprogrammed 209, the above programming method with self-boost isrepeated. If the memory cell is verified to the target threshold voltage209, the programming method is complete 211.

FIG. 3 illustrates a cross-sectional view of one embodiment of a portionof one series string in accordance with the memory array portion of FIG.1 and the programming method of FIG. 2. This figure again assumes thatembodiment 2 of the above table is used.

The selected cell 301 is shown coupled to WLN and biased at Vpgm. Theadjacent cell 302 in the same series string is coupled to WL(N−1) thatis biased at biasL. The next cell 303 in the same series string iscoupled to WL(N−2) that is biased at ground potential. This cell 303 isthus turned off. The next two cells 304, 305 are coupled to WL(N−3) andWL(N−4), respectively. These word lines are biased at V_(low) in orderto generate an area of lower self-boost 300 that reduces thepunch-through effect by reducing the voltage drop across the turned-offcell 303.

FIG. 4 illustrates a functional block diagram of a memory device 400.The memory device 400 is coupled to a processor 410. The processor 410may be a microprocessor or some other type of controlling circuitry. Thememory device 400 and the processor 410 form part of a memory system420. The memory device 400 has been simplified to focus on features ofthe memory that are helpful in understanding the present embodiments.

The memory device includes an array of flash memory cells 430 or someother type of non-volatile memory cells. The memory array 430 isarranged in banks of rows and columns. The control gates of each row ofmemory cells is coupled with a word line while the drain and sourceconnections of the memory cells are coupled to bit lines. As is wellknown in the art, the connection of the cells to the bit lines dependson whether the array is a NAND architecture, a NOR architecture, an ANDarchitecture, or some other array architecture.

An address buffer circuit 440 is provided to latch address signalsprovided through I/O circuitry 460. Address signals are received anddecoded by a row decoder 444 and a column decoder 446 to access thememory array 430. It will be appreciated by those skilled in the art,with the benefit of the present description, that the number of addressinput connections depends on the density and architecture of the memoryarray 430. That is, the number of addresses increases with bothincreased memory cell counts and increased bank and block counts.

The memory device 400 reads data in the memory array 430 by sensingvoltage or current changes in the memory array columns using senseamplifier/buffer circuitry 450. The sense amplifier/buffer circuitry450, in one embodiment, is coupled to read and latch a row of data fromthe memory array 430. Data input and output buffer circuitry 460 isincluded for bi directional data communication, as well as addressinput, over a plurality of data connections 462 with the processor 410.Write circuitry 455 is provided to write data to the memory array.

Control circuitry 470 decodes signals provided on control connections472 from the processor 410. These signals are used to control theoperations on the memory array 430, including data read, data write, anderase operations. The control circuitry 470 may be a state machine, asequencer, or some other type of controller. The control circuitry 470is adapted to execute the embodiments of the programming operation withself-boost as discussed previously.

The non-volatile memory device illustrated in FIG. 4 has been simplifiedto facilitate a basic understanding of the features of the memory and isfor purposes of illustration only. A more detailed understanding ofinternal circuitry and functions of non-volatile memories are known tothose skilled in the art.

Conclusion

One or more embodiments of the present disclosure provide a reduction inpunch-through disturb effect. By reducing the word line bias voltage onthe unselected word lines on the source-side of a turned-off word line,the voltage drop across the turned-off word line is reduced. The wordlines between the turned-off word line and the selected word line arebiased at a voltage between V_(low) and V_(pass).

Although specific embodiments have been illustrated and describedherein, it will be appreciated by those of ordinary skill in the artthat any arrangement that is calculated to achieve the same purpose maybe substituted for the specific embodiments shown. Many adaptations ofthe disclosure will be apparent to those of ordinary skill in the art.Accordingly, this application is intended to cover any adaptations orvariations of the disclosure.

1. A method for reducing punch-through effect during a programmingoperation in a memory device, the method comprising: biasing a controlgate of a selected memory cell with a program voltage; biasing controlgates of unselected memory cells on a drain-side of the selected memorycell with a V_(pass) voltage; and biasing control gates of unselectedmemory cells on a source-side of the selected cell with a V_(low)voltage that is less than the V_(pass) voltage.
 2. The method of claim 1wherein biasing the control gate of the selected memory cell comprisesgenerating a series of incrementally increasing programming pulses. 3.The method of claim 1 wherein V_(low) is about 4V less than V_(pass). 4.The method of claim 1 wherein V_(low) is in a range of 0V to 6V.
 5. Themethod of claim 1 and further including biasing a control gate of amemory cell adjacent to the selected memory cell on the drain-side ofthe selected memory cell with a first voltage that is less than V_(pass)and greater than V_(low).
 6. The method of claim 1 and further includingbiasing a control gate of a memory cell adjacent to the selected memorycell on the source-side of the selected memory cell at ground potential.7. A method for reducing punch-through effect during programming of amemory block in a non-volatile memory device, the method comprising:biasing a selected word line of a series string with at least oneprogramming pulse at an initial programming voltage; biasing, with afirst voltage, a first unselected word line on a drain-side of andadjacent to the selected word line and a second unselected word line ona source-side of and adjacent to the selected word line; biasingremaining unselected word lines on the drain-side of the selected wordline at V_(pass) that is greater than the first voltage; biasing a thirdunselected word line, adjacent to the second adjacent word line, atground potential; and biasing remaining unselected word lines on thesource-side of the third unselected word line at V_(low) that is lessthan both V_(pass) and the first voltage.
 8. The method of claim 7 andfurther including biasing the selected word line with additionalprogramming pulses if a memory cell being programmed does not reach atarget threshold voltage wherein the additional programming pulses areeach incremented by a step voltage prior to biasing.
 9. The method ofclaim 7 wherein the first voltage is in a range of 6V to 10V.
 10. Amethod for reducing punch-through effect during programming of a memoryblock in a non-volatile memory device, the method comprising: biasing aselected word line of a series string of memory cells with at least oneprogramming pulse at an initial programming voltage; biasing, with afirst voltage, a first unselected word line on a source-side of andadjacent to the selected word line; biasing unselected word lines on thedrain-side of the selected word line at V_(pass) that is greater thanthe first voltage; biasing a second unselected word line, adjacent tothe first adjacent word line, at ground potential; and biasing remainingunselected word lines on the source-side of the second unselected wordline at V_(low) that is less than both V_(pass) and the first voltage.11. The method of claim 10 wherein V_(pass) is in a range of 9V to 10V.12. The method of claim 10 wherein the series string of memory cells isa NAND architecture series string.
 13. A method for reducingpunch-through effect during programming of a memory block in anon-volatile memory device, the method comprising: biasing a selectedword line of a series string of memory cells with a programming pulse ata programming voltage; biasing, at a V_(pass) voltage, a firstunselected word line on a source-side of and adjacent to the selectedword line; biasing unselected word lines on the drain-side of theselected word line at the V_(pass) voltage; biasing, at a first voltagethat is less than the V_(pass) voltage, a second unselected word line onthe source-side of and adjacent to the first unselected word line;biasing, at ground potential, a third unselected word line, adjacent toand on the source-side of the second unselected word line; and biasing,at V_(low) that is less than both V_(pass) and the first voltage,remaining unselected word lines on the source-side of the thirdunselected word line.
 14. The method of claim 13 wherein V_(low) is 4Vless than V_(pass) and V_(pass) is in a range of 9V to 10V.
 15. Themethod of claim 13 and further including performing a programverification operation to determine if a threshold voltage of a selectedmemory cell has reached a target threshold voltage.
 16. A non-volatilememory device comprising: an array of memory cells wherein rows ofmemory cells are coupled to word lines and series strings of memorycells are coupled to bit lines; and memory control circuitry coupled tothe array of memory cells and adapted to perform a programming operationon a target memory cell that includes a self-boost operation to reducepunch-through disturb in response to a bias on a selected word line witha program voltage, a bias on unselected word lines on a drain-side ofthe selected word line with a V_(pass) voltage, and a bias on unselectedword lines on a source-side of the selected word line with a V_(low)voltage that is less than the V_(pass) voltage.
 17. The memory device ofclaim 15 wherein the non-volatile memory device is a NAND flash memorydevice.
 18. The memory device of claim 15 wherein the memory controlcircuitry is further adapted to bias an adjacent word line on thesource-side of the selected word line at a ground potential.
 19. Thememory device of claim 15 wherein the memory control circuitry isfurther adapted to bias the selected word line with a series ofincrementally increasing programming pulses in response to results of aprogram verification operation.
 20. The memory device of claim 15wherein the memory control circuitry is further adapted to turn off anadjacent memory cell to the target memory cell and bias remainingunselected word lines on the source side of the adjacent memory cellwith the V_(low) voltage.